Boilers Installation London :What is water flow rate and water pressure

About Water Pressure and Water Flow

Boilers installation London:Water pressure and flow rate which, though interdependent, are not the same. It's a common misunderstanding.

The flow rate is proportional to the resistance to flow and to the pressure. This means that:  
(a) for a given pressure, the flow rate is dependent on the resistance to flow.

 
(b) For a given resistance to flow, the flow rate is dependent on the pressure.

The explanation detailed below is based upon simple Combi boilers. Modern combis of modest power output and many older combis would fall into this category. As detailed elsewhere in this site, higher powered combis and more sophisticated combis are now available which have performance characteristics much closer to those of a conventional boiler, and thus the differences outlined below will be far less noticeable. I have used simple Combi system for the purposes of this explanation since it is with these systems that the differences are most exaggerated.

To compare hot and cold water for the same system, we need to refer to point (a).

In a Combi system, both the hot and cold water will be fed from the mains and will be at mains pressure. However the resistance to flow in the hot and cold supplies is different. The resistance due to the pipework will be more or less the same for both. But the boiler itself restricts the flow rate of hot water so that the water remains in the boiler long enough to be heated to the correct temperature. Therefore, although the pressure at the hot and cold taps is the same, the flow rate from the hot tap will be less than from the cold tap. That is the difference you notice.

Boilers installation London:In a conventional system with a hot water cylinder fed from the cold water tank in the attic, the pressure will again be the same at the hot and cold taps. The resistance to flow will be similar in both the hot and cold system and thus the flow rates will be more or less the same. The pressure of the water in this case will be due to the difference in height between the cold water tank and the tap you are using. This pressure will be considerably less than mains pressure, an important point when it comes to showers and baths (see below).

You will get a better shower with a Combi than from a conventional system.

To compare a shower fed by a Combi boiler with one fed by a hot water cylinder, we need to consider point (b) above.

The shower head will provide a large resistance to flow. With a Combi boiler the hot water is at mains pressure so the flow rate to the shower will be good (but will never be higher than the boiler itself will allow). With a hot water cylinder, the pressure is significantly lower than mains pressure so the flow rate will be comparatively low

To compare a bath fed by a Combi boiler with one fed by a hot water cylinder we need to refer to both (a) and (b).

Bath taps do not give a huge restriction to flow. Therefore from a hot water cylinder we get a reasonable flow rate despite the low pressure. However, with a Combi boiler, although the pressure is higher, the flow resistance (due to the boiler) is also much higher, and this more than offsets the higher pressure. The flow rate is therefore much lower.

To sum up: 
Combi systems 
1) high (mains) pressure hot water 
2) Good flow rate for showers 
3) Poor flow rate for baths

conventional systems 
1) low pressure hot water 
2) poor flow rate for showers (can be improved with a pump) 
3) Good flow rate for baths

Boilers installation London;Note that I have assumed a simple basic Combi boiler; a simple hot water tank in an upstairs airing cupboard with cold water tank in the attic; decent mains water pressure; houses that are not 10 stories high!!! All those parameters can vary (e.g. you have a fairly high powered Combi) and as such the differences I've outlined will be more marked in some situations than others.

To come back (finally!!) to the original topic....

As far as the choice between boiler types goes, it's a question of personal priorities. Someone (like myself) who takes mostly showers and only uses the bath for a long leisurely soak now and then, might consider the high pressure shower a considerable benefit and the slow filling bath no more than a minor irritation at most, and therefore be swayed towards a Combi system. On the other hand someone who has few showers (or maybe has a high powered electric shower already) and for whom a slow filling bath is a major irritation, might be swayed towards a conventional system.

There are, however, several other factors to consider, and again, much of it boils down to personal preference.